Granted the nuance or this political system is mostly lost on me but what harm would be done if republicans stop backing Trump?
i get they all likely fear not be re-elected and want seats at the table should the GOP get a person in the Oval Office. However, only a fool could think Trump is or more importantly should become president, so I think it is career suicide to back him more than it is to distance yourself from him. Jump ship while you can to save face, because Trump could potentially salt the earth the GOP walks on.
It's twofold, like you mentioned.
A party that isn't unified will lose the support of people who lose belief in that the party can listen to them. If the GOP don't support Trump, then Trump won't win. But the GOP can save face, but it will fracture the party because you do have some Republicans legitimately backing Trump. If the GOP support Trump, and he loses, then that will taint all the people who supported him in the future. "But you supported this racist bigot, why should we support you in the future?". Basically, anyone who isn't a Republican, likely won't become a Republican in the future until the current GOP is eradicated. And they also lose those who were Republicans but don't agree with the racism.
If Trump wins(unlikely), then he and the GOP will be at an impasse since Trump will most likely dislike and disagree with the GOP. Basically, not unified.
So what you have is the main figure heads of the GOP 'endorsing' trump. But what's happening is, Trump isn't being reined in. You have those same figure heads having to put out statements that disagree with Trump, but not directly. Trump himself, dislikes the GOP. To an outsider, it all looks like a big fat mess. To people who align with the GOP? They're seeing a party that can't stick together and that won't come together to help each other. So they become disillusioned, and you're seeing some GOP denounce Trump publicly or say they'll vote for Hillary.
The party is dying a fast death. The Southern Strategy, with all their racist dog whistles, what was a slow death, has been sped up by Trump.
It's...astounding. There will be books written about this election.